Raised Bed Soil Mix Calculator

How it works

A raised bed is a box, so the soil it holds is its inside length × inside width × fill depth. The fill depth is the bed height less the headspace left unfilled below the rim. For several beds of the same size the volume is multiplied by the number of beds. Dividing cubic feet by 27 gives cubic yards, the unit bulk soil is sold in.

The blend splits that volume by ratio. Mel's Mix, from Mel Bartholomew's Square Foot Gardening method, is equal thirds of blended compost, peat moss (or coir) and coarse vermiculite. The 60/30/10 blend is 60% topsoil, 30% compost and 10% aeration material such as vermiculite or perlite. Custom accepts any four percentages that add up to 100.

Each ingredient's volume is divided by the bag size and rounded up to whole bags. Peat moss is also sold as compressed bales that expand when loosened; the bale count divides the peat volume by the loose yield of one bale.

Formula

fill depth   = bed height − headspace                    (in)
cubic feet   = beds × length × width × fill depth ÷ 12
cubic yards  = cubic feet ÷ 27
ingredient   = cubic feet × share
bags         = ⌈ ingredient ÷ bag size ⌉
peat bales   = ⌈ peat volume ÷ loose yield per bale ⌉
Mel's Mix: ⅓ compost + ⅓ peat or coir + ⅓ vermiculite
60/30/10:  60% topsoil + 30% compost + 10% aeration

Example

One 8 ft × 4 ft bed 12 in high, filled to 1 in below the rim, holds 32 sq ft × 11⁄12 ft = 29.33 cu ft, or 1.09 cu yd.

In Mel's Mix that is 9.78 cu ft each of compost, peat or coir, and vermiculite: 7 bags of each at 1.5 cu ft, or 2 compressed peat bales at a typical 7.5 cu ft loose yield instead of the peat bags.

The 60/30/10 blend for the same bed is 17.6 cu ft of topsoil (12 bags at 1.5 cu ft), 8.8 cu ft of compost (6 bags) and 2.93 cu ft of aeration material (2 bags).

Assumptions and limitations

• Dimensions are inside measurements and the bed has a flat bottom and straight sides. Boards, posts and any wedge-shaped or tapered sides are not modelled.
• Ingredient volumes are added as if they did not shrink when mixed. Fine particles fill gaps between coarse ones, and organic matter settles after watering, so a mixed bed usually ends up somewhat lower than the loose volumes suggest; beds are commonly topped up in later seasons.
• Bag sizes are nominal volumes as packed. The peat bale yield is a typical figure and is editable; the bale label states the yield for a particular product.
• The blends are common published recipes, not soil-test recommendations. Mel's Mix proportions are from the Square Foot Gardening method; the 60/30/10 split is a widely used typical ratio and can be changed with the custom option.
• Drainage material, liners and any soil already in the bottom of the bed are not included.

Frequently asked questions